Sealing and conducting gasket material



May 5, 1959 'v. PULSIFER ETAL 2,885,459 SEALING AND cououcwmc GASKETMATERIAL Filed Nov. 2, 1955KLKMWKMKM((GKMKWKKW(((((((((((((W((XK(((((U((Km J5 Q r WW q UnitedStates Patent C) SEALING AND CONDUCTING GASKET MATERIAL Verne Pulsifer,La Grange, and Andrew H. Murphy, Chicago, 111., assignors to the UnitedStates of America as represented by the Secretary of the Air Force Thisinvention relates to gasket materials and more particularly to a gasketmaterial which will both provide a gas tight seal at a joint and willprevent electrical energy of radio frequencies from escaping at jointsin closed containers and to a method for making the gasket material.

As a background for insuring a sufiicient understanding of the presentinvention as claimed, the transmission of electromagnetic energy alongand its confinement by the metal walls of a wave guide of the like,includes propagation in angular and in radial modes with lines of forcewhich emerge perpendicularly from the guide wall, which turn and travelparallel to the guide axis and then bend back into the guide. Bothelectrical and magnetic modes may be directed to be transverse to theaxis of the wave guide in their propagation therealong.

When an electromagnetic wave is incident to a conducting metal surface,such as the inside of a metal container, electrical and magnetic energycurrents are set up in the metal. If the metal were a perfect conductorit might be assumed that a field equal and opposite to the excitingfield would be set up at the metal surface in a vanishingly small skindepth. Incident energy would be partly reflected and would be partlyretained in the metal container. Since the metal has finiteconductivity, there is current flow in the metal and a portion of theincident energy is retained within the container. The current flow inthe metal increases its temperature and part of the current passesthrough the metal and is radiated. Therefore, of the energy incident ona metal surface, part is reflected, part is attenuated within the metaland part of the energy escapes as radiation. Energy attenuation is afunction of the permeability of the metal.

Energy of very high radio frequencies, above a million cycles persecond, incident to good conductors is very nearly all reflected andvery little is attenuated or is radiated. It follows, therefore, thatthe mechanism of the shielding of energy at frequencies above a millioncycles per second is principally one of reflection. The conduction ofelectromagnetic energy of frequencies above a million cycles per secondand past a joint or other discontinuity between abuting ends of twosections of a wave guide without loss, the present invention isconcerned. Two pipe flanges abutted together comprise opposed flangesurfaces which act as a radial Wave guide in transmitting radiofrequency energy to the outside of the pipe from which it is radiated.

A smoothly soldered or welded junction between the abutting ends of twowave guide sections is in effect a continuous metal wall and minimizeslosses in electromagnetic energy at the junction. Flanged pipe ends maybe smooth faced and clamped together with bolts. It might be assumedthat such a joint would function with negligible loss of radio frequencyenergy if the flanged surfaces were substantially smooth and free fromirregularities or projections on opposed pairs of surfaces so boltedtogether.

Irregularities on finely turned copper are about x10 is a problem withwhich serving to minimize interference in neighboring sensiice cm. high.Irregularities on optically polished aluminum are between and 1,000angstroms in height. Irregularities on steel surfaces lapped with gradecarborundum paper are about 5 10- cm. high and steel surfaces polishedwith 600 grade carborundum paper are about 10- cm. high. In practicalapplication flange surfaces are far more irregular than polishedsurfaces and touch at but few points with open gaps at most of thejoint. The irregularities, as energy radiators, objeotionably dissipatethe energy conducted by the wave guide and objectionably raise theinterference level in the operation of adjacent other sensitiveelectronic equipment. Continuously forming oxide films on flangesurfaces'further reduce radio frequency conductivity. Very highpressures capable of penetrating oxide films are not practical onabutting flange surfaces. I

In an effort to minimize electromagnetic energy losses at wave guidejunctions and the like, and further to provide a gas-tight seal thereat,a number of available gaskets and sealing arrangements have been testedat constant energy levels of 3 and 3.5 amperes over a frequency rangeof'from 150 kilocycles to 150 megacycles and with flange pressures offrom 50 to 200 pounds per square inch. Shielding and sealing materialsconsisting of metal screen, wire cloth and the like, imbedded in rubberor in paste, were tested experimentally and information gained from theinvestigation was used in arriving at the present invention. Conductorsextending parallel to the flange surfaces act as antennae to conductradio frequency electrical energy to outside of the container. Metalconductors subjected to insufficient pressure to make electrical contactwith the flange surfaces are ineffectual. In the presence of anexcessive number of conductors the pressure between the conductor endsand the flange surfaces may be insuflicient to provide adequate contact.

In this experimentation of radio frequency shielding and sealing gasketsit was determined that flange pressure is a first order factor and thatthe impedance of a gasket is without meaning unless it is accompanied bythe pressure at which the impedance is measured. The clamping pressureon bolt secured flanges may range from a ton per square inch under thebolts to 100 pounds per square inch between the bolts. The variation inpressure between bolts is large with ordinary gaskets. Low pressurepoints in ordinary gaskets originate interference in neighboringelectronic equipment. Gasket material shielding satisfactorily at lowclamping pressures is highly desirable.

A general statement of the nature and of the substance of the presentinvention, as claimed herein, comprises in nature a gasket shieldingelectromagnetically and pres sure sealing joints in hollow conductorsand in substance a compressible gasket comprising an electromagneticallyshielding resilient multiplicity of unconnected wires supported by apressure sealing resilient body and to a method for making the gasket.The word unconnected, as used herein, has its usual dictionarydefinition of separate or not joined or linked together.

A general statement object of the present invention, as claimed, is toprovide previously unobtainable improved gasket material which willprevent radio frequency current from escaping at joints in closedcontainers or to provide an electromagnetically shielding and pressuresealing gasket applicable to joints in wave guides and the like, and toa method for its production, with the gasket tive electronic equipment.

Other objects are to provide a gasket material which supplies amultiplicity of electrically conducting paths between the surfaces ofopposed metal flanges; a gasket material providing an adequate densityof mutually unconnected metal conductors with their ends exposed toenemas make adequate break-through contacts with the flange metaltocarry a predetermined current and the gasket material also providing agas impervious seal which is effective at a particular pressure; toprovide a preformed gasket having a radio frequency impedance which islower'than that of any previously available gasketv material; a gasketmaterial providing a desired density of or number per unit area of radiofrequency conduction paths evenly distributed over the gasket andmaintaining uniformly distributed high local pressure points evenlyspaced over the engaged flange surface with the pressure at each contactpoint adequately high to break through any surface film of oxides or thelike on the flange surfaces; to provide a joint sealing andelectromagnetically screening gasketiwhich is gas and weather tightapplicable toan installation subjected to varying. weather conditionsduring its'use through the agency ofv a rubber or similar material whichunder pressure flows, easily to-fill gaps and bs el s 9W damn n pressures us being shared betweeh the; ntacts the sealing agents in a s is a s'9 PIQi q as sealing gas e inso ia and of unimpaired service in thepresence of gasoline, oil and water and that bonds well to metal over atemperature range of front about 65 F. to 400 F. in which range aircraftengines are operated; to provide a shielding and a sealing gasket havingappreciable func-. tional irnprovernents'over previously availablecomparable gaskets made of woven wire, screen wire, and the like inneoprene rubbenmatted copper fiber, in cements, chromium granulesfcopperfiber andthe like; to provide a shielding and a sealing gasket having apeak impedance curve at'about 170 conductors per. square inch and aminimum impedahce value at about 375 conductor paths per square inch'atapressure of about 200 pounds per square inch using a frequency in therange of from 0.15 to megacycles per second; andto provide a shieldingand a sealing gasket comprising silicone rubber and unconnected metalfibers running at an angle from one surface to the other surface of therubber to provide a gas tight seal between flange surfaces underconditions of high temperature; and to the method of making the gasket.i

An illustrative embodiment of the present invention is represented inthe accompanying drawing wherein:

Fig. 1 is a plan view of thesurface of a gasket material which embodiesthe present invention;

Fig. 2 is an edge elevational view of the gasket in Fig. 1 showingunattached wires slightly. bent between their ends'and embedded insilicone rubber; l t

'Fig. 3 is an enlargedschematic diagram of the gasket wires or metalfibersinjthrustbetweenopposed flange faces, with the flange rubbernot'show n in section [or purposes of clarity, and with curved linesindicating electromagnetic radiation energy radiated from the left ofthe joint from insidea hollow conductor and of reduced density. towardthe right, from the action of the flange bridging wires or metal fibers,to no radiation loss at the flange outside to the right of the hollowconductor; and i Fig. 4 is an enlarged schematic diagram of gaskethaving as conductors straight wires or, metal fibers making angles fromone sidefof the gasket to the other in acrisscross pattern in the gasketrubber body portion, which again is not shown' in section for purposesof clarity.

The gasket material embodying the present invention and represented inthe accompanying drawing, comprises a resilient material, such as.rubber material 1 or the like, in which are embedded a multiplicity ofelectrically,

conducting metal'fibers or ivires'2 which are embedded within and whichare inclined to the planes'of opposite sides'of the resilie'ntinateri'al1L The'wires illustratively may be of stainless steel or. the like, andthe resilient material 1 illustratively may be of silicone rubber or anequivalent material. The density of thewi re distribution in the rubberpreferably is about from .250 to. 500 wires 4 per square inch, althoughthe invention functions experimentally outside of this range.

With the gasket material positioned between a pair of opposed flangefaces in a wave guide, the wires 2'; are maintained yieldingly in thrustso that they are curved somewhat between their ends, represented in thedrawing as the curve 3. The wires 2 are of a diameter or a gage and of acomposition which are commensurate with the pressure to which the gasketis to be subjected and to the service to which the gasket is to be put.The wires 2 may, as a primed modification illustrated in Fig. 4, bestraight wires making angles from one side of the gasket to the other ina crisscross pattern in the rubber within which they are embedded.

Optimum radio frequency shielding effects are accomplished when anadequate number of direct electrical conducting paths are providedbetween and in direct contact with opposed metal flange faces 4 and 5between which the gasket material is positioned.

In making the gasket material, the gasket rubber body portion iscalendered in usual manner between rolls and then a multiplicity ofsteel wires is thrust through it at regularly spaced intervals, so thateach wire completely penetrates the body or base material and projectsslightly from the opposite surfaces thereof. The preferred slight bendbetween the ends of each wire imparts to it a spring action undercompression rather than a stiff strut action. The wire ends. on oppositefaces of the gasket rubber portion are then clipped to uniform length.The sheets of gasket material so made may then be cut to desiredfigurations and dimensions in the making of manufactured, standardizedgaskets. The slight bend in each wire, or the angle at which the wiresare positioned in the base material or both impart a spring action tothe wire by which good contact is secured between the wire ends and theflange surfaces under normal clamping pressures to which the gasket issubjected.

It has been determined experimentally that commercially available cardcloth used in processing cotton fibers may be modified for itsapplication and use as one form of the gasket contemplated hereby. Cardcloth used in processing cotton has a heavy fabric backing of canvas orthe like on one side of a rubber base material with illustratively from250 to 500 steel staples driven through the base material atregularlyspaced intervals so that the points of the staples project at an anglefrom the surface-ofthe card cloth not covered with canvas.

In adapting commercially available card cloth to one form of the presentinvention, thecanvas back and a portion of the, staples may be removedby grinding or by a similar operation, tov provide. a sheet of rubberbase material for the, multiplicity of unconnected small steel Wiresextending through the base. material and presenting exposed Wire endsfrom both ofthe opposite-sides therof.

Following; the above described method of making gasket material fromavailable card cloth, the exposed wire staple ends are cut to a uniformlength closely adjacent to the surface of the rubber by which they aresupported. The gasket material is then out to desired shapes to fitjoints. with which. it is to be used in sealing the joints against bothgas pressure and the loss therethrough of radio frequencyelectromagnetic energy.

satisfactorily operatinggaskets embodying the present invention were sthick and had about 500 bridging wires per square inch with, each wire.005" in diameter. A second gasket Was AM thick and also had about 500bridging wires per square. inch, but with each wire 0.020 in diameter.The wire. density or the. number of wires per square inch may. be.varied over a wide range depending upon the, degree of shieldingwhich itis desired to accomplish-andthe magnitude of clamping pressure which isrequired for aparticular installation.

A prefer red;method of making gasket material embodylhg the presentinvention is the passingof a rubber sheet of the required thicknessthrough a stapling machine where a desired multiplicity of wires areinserted into and left in the rubber sheet and then passing the stapledmetal sheet between two grinding wheels which reduced the wires exposedon both sides of the sheet to a uniform length.

An excellent gasket material may be made in sheet form from card clothby moulding, spraying or dipping the card cloth in rubber and thencutting away excess canvas wire or rubber to make a gasket materialwhich is substantially that represented in Figs. 1 and 2 of theaccompanying drawing.

The shielding effectiveness of the described gasket material isdetermined by measuring its impedance at various clamping pressures.Experimental impedance values of the described gasket material at afrequency of 2.6 megacycles was ohms. This impedance is a four magnitudeimprovement by the gasket material which embodies the present invention,as compared with the best commercially available gasket material uponwhich experiments were made which measured an impedance at the frequencyof 2.6 me. as 10- ohms.

A gasket to seal at a relatively low clamping pressure may be made of asoft rubber measuring about 40 to 50 on a durometer. Gaskets preferablyare as thin as possible with optimum performance of shielding andsealing. Constant energy level tests indicate that shielding is moredifiicult in the frequency region of 1.5 megacycles than at adjacentupper and lower frequencies.

The characteristics of a satisfactory shielding and sealing gasketembodying the present invention are: that the conduction paths acrossthe joint occupied by the gasket should not be connected with eachother; that the radio frequency conducting wires should be substantiallyevenly distributed within the gasket material; the wires should be goodconductors of electrical energy and should be chemically inert to anymaterial in the gasket; the wires should be sufliciently hard temperedso that their tips may break through or penetrate any surface coat onthe opposed flange surfaces to make firm electrical contact therewithwhen the junction is under pressure; and when under pressure the wiresshould uniformly yieldingly oppose that pressure; and the sealing bodyof the gasket should provide a gas tight and a permanently weather tightseal at the joint.

The number of electrically conducting paths per square inch is a factorin controlling local contact pressures to which the gasket is subjected.There is an optimum number of conductors per square inch for eachcombination of gasket design for conducting material and the mechanicalproperties of the electrical conducting metal elements and of the gasketbody sealing agent under an established clamping pressure are such as toestablish and to maintain satisfactory electrical conducting pathssimultaneously with the gasket body portion providing an adequate gasseal at the same clamping pressures.

It has been established experimentally that gaskets embodying thepresent invention and mounted between flanges made of alumium or ofmagnesium perform nearly as well as when between flanges made of brass.This experimentation indicates that aluminum and magnesium also mayserve as shielding materials, when joined with gaskets embodying thepresent invention.

It is to be understood that the composition of the gasket body and thecomposition, the distribution and the characteristics of the wiressupported by the gasket body may be modified and substitutions may bemade therein within the scope of the present invention without departingfrom the functional advantages thereof.

We claim:

1. A method for making gasket material, comprising the steps of forminga resilient material into a sheet of gasket body portion, inserting intothe gasket body portion a plurality in the order of about 250 to 500wires per square inch of isolated and unconnected slightly bent wires toextend transversely therethrough and to establish substantially uniformlength of wires with wire ends exposed from opposite sides of the gasketbody portion to assume a predetermined related mechanical compressionresistance between the wires and the gasket body portion.

2. An electrically conducting gasket for minimizing the electricalimpedance between joint mating surfaces of a hollow electrical shieldhousing a radio frequency energy conductor, the gasket comprising aresilient gasket material to be interposed between the joint matingsurfaces to provide pressure and moisture sealing engagement therewith,and a plurality of electrically conductive wires supported by theresilient gasket material with each wire electrically and mechanicallyisolated from all other wires in the structure for the purpose ofmaintaining a plurality of separate and discrete low impedanceelectrical paths between the joint mating surfaces of the hollowelectrical shield and for preventing the conduction of radio frequencyenergy from the interior of the hollow electrical shield to the exteriorof the gasketed joint.

3. The gasket defined in the above claim 2 wherein each wire has aslight bend between its ends for imparting spring action to the wire indelivering high unit contact pressure between the wire ends and theflange surfaces of the joint mating members.

References Cited in the file of this patent UNITED STATES PATENTS1,924,622 Norton Aug. 29, 1933 2,125,378 Kadas Aug. 2, 1938 2,477,267Robinson July 26, 1949 2,674,644 Goodloe Apr. 6, 1954 2,686,891 BurginAug. 17, 1954

